I am not sure how you set up your project, but the debugger needs to be pointed at the executable, not the application package. Try using "/Users/john/dev/projects/AppName/build/Debug/AppName.app/Contents/MacOS/AppName"

We do not yet have support for LLDB, but that is planned for a future release.

Would you be able to share your Xcode project with us, I might be able to tweak things to feed the correct executable to GDB. You could also use Debug > Attach Debug > Debug Other Executable... and point it to the executable.

I did get a warning about required elevated privileges when I first tried debugging so I'm not sure that it's related to that.

I also tried it no breakpoint set. Once it was running I selected a breakpoint and got the program to that point (from what I can tell) and I can step through the lines of the code fine but not able to see the values of any variables. I suspect that won't be available soon.

In any event when it does finally fully support LLDB and Swift I think it will be a top contender for developing Swift apps due to editor functionality and the debugger.

I was able to step through a simple Swift project and set and hit breakpoints with GDB, those thing worked pretty well. It could evaluate simple (integer) local variables. Globals and function names were pretty badly mangled, it looks like GDB has no clue about Swift name mangling at this early stage, maybe a future release of GDB will. Swift string variables are opaque, so you couldn't really see their contents, though you could drill down far enough to see their length.

LLDB integration will be a huge improvement for MacOS, but it is no trivial undertaking so for the time being, it still falls under the "future, hopefully next, release" status.